In this grant proposal, we outline studies designed to elucidate the manner in which heparin and heparin-like molecules are able to regulate specific aspects of the thrombotic and atherosclerotic processes. Section I is devoted to expanding our knowledge of the structural domains on the heparin molecule which are critically involved in the binding of the mucopolysaccharide to antithrombin as well as in the acceleration of hemostatic enzyme - protease inhibitor interactions. In addition, we hope to define regions on the antithrombin molecule that are necessary for the complexing of the mucopolysaccharide to the protease inhibitor as well as those that are required for the dramatic enhancement in the rate of hemostatic enzyme-antithrombin interactions. Furthermore we expect to establish a potentially new role for the anticoagulantly active heparin-like species as a modulator of the fibrinolytic system via its action on endothelial cells. Section II focuses upon extending our findings which suggest that anticoagulantly active heparin-like molecules regulate coagulation system activity. To evaluate this hypothesis we will perform perfusion studies in rat/primate models and determine the relative importance of the heparin-antithrombin and the Protein C-thrombomodulin mechanisms in the modulation of hemostatic system activity. In addition, cloned rat microvascular endothelial cells will be employed to ascertain whether these cellular elements are able to synthesize anticoagulantly active heparin-like molecules. The structure of anticoagulantly active mucopolysaccharide liberated from the vascular tree and that isolated from cultured microvascular endothelial cells will be directly established. Furthermore, we will evaluate the physiologic functioning of the heparin-antithrombin and the Protein C-thrombomodulin mechanisms in humans who are generating high levels of thrombin within their circulatory systems. Section III is directed at evaluating the role of heparin-like molecules in suppressing the growth of smooth muscle cells within the vessel wall, establishing the structure-function relationships responsible for this phenomenon, identifying cellular elements which synthesize antiproliferatively active mucopolysaccharide, and characterizing the fashion by which these molecular species are able to oppose the action of mitogenic factors.